As a construction machine that has a boom and an arm, for example, a hydraulic shovel described in Patent Document 1 is known.
The hydraulic shovel described in Patent Document 1 includes a boom cylinder that causes a boom to perform rising operation or lowering operation, an arm cylinder that causes an arm to perform pushing operation or pulling operation, a first hydraulic pump, and a second hydraulic pump.
Furthermore, the hydraulic shovel includes a control valve that belongs to a first group for controlling supply and discharge of hydraulic oil from the first hydraulic pump with respect to the boom cylinder and the arm cylinder, and a control valve that belongs to a second group for controlling supply and discharge of hydraulic oil from the second hydraulic pump with respect to the boom cylinder and the arm cylinder.
Specifically, the first and second groups include the control valve for a boom for controlling supply and discharge of hydraulic oil with respect to the boom cylinder, and the control valve for an arm for controlling supply and discharge of hydraulic oil with respect to the arm cylinder, respectively.
The control valve for a boom and the control valve for an arm have a center bypass passage connected in series by a tandem line, respectively. Additionally, the control valve for a boom and the control valve for an arm are connected in parallel to a first pump via a parallel circuit.
In the hydraulic shovel, arm pulling, and boom rising that is operation with a relatively higher load than the arm pulling are sometimes combined to be operated. In this combined operation, in order to suppress the supply of the hydraulic oil from the pumps only to the arm cylinder on the lower load side, the parallel circuit that belongs to the first group is provided with a restrictor.
Consequently, in the combined operation of arm pulling and boom rising, the hydraulic oil from the first pump can be preferentially guided to the boom cylinder.
However, the hydraulic shovel described in Patent Document 1 has a problem that the power of the first pump is lost in a case where arm pushing, and boom lowering that is operation with a relatively lower load than the arm pushing are combined to be operated.
Specifically, when the boom lowering operation is performed, an opening of the center bypass passage of the control valve for a boom is narrowed. As a result, the hydraulic oil from the pumps is guided to the control valve for an arm through the parallel circuit. However, this parallel circuit is provided with the restrictor, and therefore the hydraulic oil is guided preferentially to the boom cylinder on the lower load side than the arm cylinder. Therefore, extra hydraulic oil is supplied to the boom cylinder, thereby wasting the power of the first pump.
Particularly, the capacity of the pump can be generally controlled such that the capacity of the pump is increased in accordance with increase in a boom lowering operation amount. Therefore, the larger the boom lowering operation amount is, the larger the loss of the power is.
Patent Document 1: Japanese Unexamined Patent Publication No. 2007-23606